1. Field of the Invention
The invention relates to a transmission and reception module for a bidirectional optical communication and signal transmission, having a light transmitter with a first lens coupling optical element, a light receiver, a fiber connection with a second lens coupling optical element for a common optical waveguide, and a beam splitter disposed in a free beam path, which are all surrounded by a common housing, the light transmitter, the beam splitter and the fiber connection, as well as the light receiver which is orthogonal to them, are disposed axially symmetrically.
One such bidirectional module with free beam guidance is known from European Patent Application 0 238 977 A2, corresponding to U.S. Pat. No. 4,767,171. In that transmission and reception module for a bidirectional communication network, two spherical lenses are spaced apart and disposed essentially between a laser diode and one end of an optical fiber in order to focus the laser light on the fiber end. Disposed between the spherical lenses is a beam separator or a beam splitter, which splits light, projected from the fiber end divergently toward the lens remote from the transmitter, which light is formed into a beam by that lens, at a wavelength other than the wavelength of the laser light, from the beam path. The split path is directed it to a detector or light receiver.
A special embodiment of such a module, in which the light transmitter and the light receiver, each forming an independent component, are surrounded by a hermetic capsule and inserted into a common housing with the beam splitter, is described in European Patent Application 0 463 214 A1, corresponding to U.S. Pat. No. 5,127,075.
It is also known from European Patent Application 0 400 161 A1, corresponding to U.S. Pat. No. 5,066,089, to reduce troublesome backreflections, in a configuration for optical coupling of an electrooptical converter module with an optical waveguide, without reducing the coupling efficiency of the configuration, through the use of two lenses, the first of which is disposed upstream of the converter module and the second of which is disposed upstream of the optical waveguide. The optical axis of the first lens is axially offset from the optical axis of the converter module, the optical axis of the second lens is axially offset from the optical axis of the optical waveguide, and the optical axes of the first lens and the second lens are axially offset from one another.
It is known that the proportion of reflection, which repeatedly occurs at the optical boundary surfaces in modules with optical free beam guides, and which reduces the transported light output, can also be reduced to proportions of .ltoreq.1%, by coating the optical boundary surfaces.
Nevertheless, in an electrooptical bidirectional module, two problems of a special type still occur, which negatively affect the output capacity of the module. First, high-power laser diodes as light transmitters are extremely sensitive to the smallest proportions of negative feedback light (.ltoreq.0.01% =-40 dB) and react with increased noise. Second, in bidirectional modules and especially modules that use a wavelength, all of the reflected proportions of light (&lt;1%=&lt;-20 dB) that arrive at the optical beam path contribute substantially to undesirable crosstalk between the transmission and reception channels.
An attempt has already been made to counteract this problem by providing all of the optical boundary surfaces with the best possible, and therefore expensive, coating. However, those coatings are very difficult or expensive to apply, especially to fiber boundary surfaces. Among other reasons, the fiber boundary surfaces are in particular located precisely in the most optically sensitive focused beam region, where backreflection into the optical beam path is highest. Moreover, the reduced reflection (&lt;1%) achieved by the coating is not enough for the full power of the particular module to be reached.